Even though William was a firm ally of the British he ended up with nothing after the Piece of Amiens, the Dutch possessions occupied by the British were to be returned to the Batavian Republic and the House of Orange-Nassau was to be merely compensated for their losses (he gained a small principality in the HRE).
Now, WI there was a agreement to give him the Cape and perpetual neutrality? Both the French and British are interested in the strategic value of the Cape and it sounds like a resonable middle ground to me (comparatively Amiens also restored the Knights of Malta who were supposed to be neutral).
Would William accept such proposal? The King of Portugal later accepted to be transfered to Brazil, but he never really needed to give up his claim to Portugal. OTOH William was only the stadhouder, not really a king, so could he be happy such small proposal?
Let's say that the Cape remains neutral and Napoleon doesn't fall, thus the Netherlands remains under French influece. How's the future for the Cape Kingdom?